/*
 * Copyright (C) 2008 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.CollectPreconditions.checkNonnegative;
import static java.util.Objects.requireNonNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Function;
import com.google.common.base.Predicate;
import com.google.common.base.Predicates;
import com.google.common.math.IntMath;
import com.google.common.primitives.Ints;

import java.util.AbstractCollection;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import javax.annotation.CheckForNull;

import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Provides static methods for working with {@code Collection} instances.
 *
 * <p><b>Java 8 users:</b> several common uses for this class are now more comprehensively addressed
 * by the new {@link java.util.stream.Stream} library. Read the method documentation below for
 * comparisons. These methods are not being deprecated, but we gently encourage you to migrate to
 * streams.
 *
 * @author Chris Povirk
 * @author Mike Bostock
 * @author Jared Levy
 * @since 2.0
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
public final class Collections2
{
    private Collections2()
    {
    }

    /**
     * Returns the elements of {@code unfiltered} that satisfy a predicate. The returned collection is
     * a live view of {@code unfiltered}; changes to one affect the other.
     *
     * <p>The resulting collection's iterator does not support {@code remove()}, but all other
     * collection methods are supported. When given an element that doesn't satisfy the predicate, the
     * collection's {@code add()} and {@code addAll()} methods throw an {@link
     * IllegalArgumentException}. When methods such as {@code removeAll()} and {@code clear()} are
     * called on the filtered collection, only elements that satisfy the filter will be removed from
     * the underlying collection.
     *
     * <p>The returned collection isn't threadsafe or serializable, even if {@code unfiltered} is.
     *
     * <p>Many of the filtered collection's methods, such as {@code size()}, iterate across every
     * element in the underlying collection and determine which elements satisfy the filter. When a
     * live view is <i>not</i> needed, it may be faster to copy {@code Iterables.filter(unfiltered,
     * predicate)} and use the copy.
     *
     * <p><b>Warning:</b> {@code predicate} must be <i>consistent with equals</i>, as documented at
     * {@link Predicate#apply}. Do not provide a predicate such as {@code
     * Predicates.instanceOf(ArrayList.class)}, which is inconsistent with equals. (See {@link
     * Iterables#filter(Iterable, Class)} for related functionality.)
     *
     * <p><b>{@code Stream} equivalent:</b> {@link java.util.stream.Stream#filter Stream.filter}.
     */
    // TODO(kevinb): how can we omit that Iterables link when building gwt
    // javadoc?
    public static <E extends @Nullable Object> Collection<E> filter(
            Collection<E> unfiltered, Predicate<? super E> predicate)
    {
        if (unfiltered instanceof FilteredCollection)
        {
            // Support clear(), removeAll(), and retainAll() when filtering a filtered
            // collection.
            return ((FilteredCollection<E>) unfiltered).createCombined(predicate);
        }

        return new FilteredCollection<E>(checkNotNull(unfiltered), checkNotNull(predicate));
    }

    /**
     * Delegates to {@link Collection#contains}. Returns {@code false} if the {@code contains} method
     * throws a {@code ClassCastException} or {@code NullPointerException}.
     */
    static boolean safeContains(Collection<?> collection, @CheckForNull Object object)
    {
        checkNotNull(collection);
        try
        {
            return collection.contains(object);
        }
        catch (ClassCastException | NullPointerException e)
        {
            return false;
        }
    }

    /**
     * Delegates to {@link Collection#remove}. Returns {@code false} if the {@code remove} method
     * throws a {@code ClassCastException} or {@code NullPointerException}.
     */
    static boolean safeRemove(Collection<?> collection, @CheckForNull Object object)
    {
        checkNotNull(collection);
        try
        {
            return collection.remove(object);
        }
        catch (ClassCastException | NullPointerException e)
        {
            return false;
        }
    }

    static class FilteredCollection<E extends @Nullable Object> extends AbstractCollection<E>
    {
        final Collection<E> unfiltered;
        final Predicate<? super E> predicate;

        FilteredCollection(Collection<E> unfiltered, Predicate<? super E> predicate)
        {
            this.unfiltered = unfiltered;
            this.predicate = predicate;
        }

        FilteredCollection<E> createCombined(Predicate<? super E> newPredicate)
        {
            return new FilteredCollection<E>(unfiltered, Predicates.<E>and(predicate, newPredicate));
            // .<E> above needed to compile in JDK 5
        }

        @Override
        public boolean add(@ParametricNullness E element)
        {
            checkArgument(predicate.apply(element));
            return unfiltered.add(element);
        }

        @Override
        public boolean addAll(Collection<? extends E> collection)
        {
            for (E element : collection)
            {
                checkArgument(predicate.apply(element));
            }
            return unfiltered.addAll(collection);
        }

        @Override
        public void clear()
        {
            Iterables.removeIf(unfiltered, predicate);
        }

        @Override
        public boolean contains(@CheckForNull Object element)
        {
            if (safeContains(unfiltered, element))
            {
                @SuppressWarnings("unchecked") // element is in unfiltered, so it must be an E
                E e = (E) element;
                return predicate.apply(e);
            }
            return false;
        }

        @Override
        public boolean containsAll(Collection<?> collection)
        {
            return containsAllImpl(this, collection);
        }

        @Override
        public boolean isEmpty()
        {
            return !Iterables.any(unfiltered, predicate);
        }

        @Override
        public Iterator<E> iterator()
        {
            return Iterators.filter(unfiltered.iterator(), predicate);
        }

        @Override
        public boolean remove(@CheckForNull Object element)
        {
            return contains(element) && unfiltered.remove(element);
        }

        @Override
        public boolean removeAll(final Collection<?> collection)
        {
            boolean changed = false;
            Iterator<E> itr = unfiltered.iterator();
            while (itr.hasNext())
            {
                E e = itr.next();
                if (predicate.apply(e) && collection.contains(e))
                {
                    itr.remove();
                    changed = true;
                }
            }
            return changed;
        }

        @Override
        public boolean retainAll(final Collection<?> collection)
        {
            boolean changed = false;
            Iterator<E> itr = unfiltered.iterator();
            while (itr.hasNext())
            {
                E e = itr.next();
                if (predicate.apply(e) && !collection.contains(e))
                {
                    itr.remove();
                    changed = true;
                }
            }
            return changed;
        }

        @Override
        public int size()
        {
            int size = 0;
            for (E e : unfiltered)
            {
                if (predicate.apply(e))
                {
                    size++;
                }
            }
            return size;
        }

        @Override
        public @Nullable
        Object[] toArray()
        {
            // creating an ArrayList so filtering happens once
            return Lists.newArrayList(iterator()).toArray();
        }

        @Override
        @SuppressWarnings("nullness") // b/192354773 in our checker affects toArray declarations
        public <T extends @Nullable Object> T[] toArray(T[] array)
        {
            return Lists.newArrayList(iterator()).toArray(array);
        }
    }

    /**
     * Returns a collection that applies {@code function} to each element of {@code fromCollection}.
     * The returned collection is a live view of {@code fromCollection}; changes to one affect the
     * other.
     *
     * <p>The returned collection's {@code add()} and {@code addAll()} methods throw an {@link
     * UnsupportedOperationException}. All other collection methods are supported, as long as {@code
     * fromCollection} supports them.
     *
     * <p>The returned collection isn't threadsafe or serializable, even if {@code fromCollection} is.
     *
     * <p>When a live view is <i>not</i> needed, it may be faster to copy the transformed collection
     * and use the copy.
     *
     * <p>If the input {@code Collection} is known to be a {@code List}, consider {@link
     * Lists#transform}. If only an {@code Iterable} is available, use {@link Iterables#transform}.
     *
     * <p><b>{@code Stream} equivalent:</b> {@link java.util.stream.Stream#map Stream.map}.
     */
    public static <F extends @Nullable Object, T extends @Nullable Object> Collection<T> transform(
            Collection<F> fromCollection, Function<? super F, T> function)
    {
        return new TransformedCollection<>(fromCollection, function);
    }

    static class TransformedCollection<F extends @Nullable Object, T extends @Nullable Object>
            extends AbstractCollection<T>
    {
        final Collection<F> fromCollection;
        final Function<? super F, ? extends T> function;

        TransformedCollection(Collection<F> fromCollection, Function<? super F, ? extends T> function)
        {
            this.fromCollection = checkNotNull(fromCollection);
            this.function = checkNotNull(function);
        }

        @Override
        public void clear()
        {
            fromCollection.clear();
        }

        @Override
        public boolean isEmpty()
        {
            return fromCollection.isEmpty();
        }

        @Override
        public Iterator<T> iterator()
        {
            return Iterators.transform(fromCollection.iterator(), function);
        }

        @Override
        public int size()
        {
            return fromCollection.size();
        }
    }

    /**
     * Returns {@code true} if the collection {@code self} contains all of the elements in the
     * collection {@code c}.
     *
     * <p>This method iterates over the specified collection {@code c}, checking each element returned
     * by the iterator in turn to see if it is contained in the specified collection {@code self}. If
     * all elements are so contained, {@code true} is returned, otherwise {@code false}.
     *
     * @param self a collection which might contain all elements in {@code c}
     * @param c    a collection whose elements might be contained by {@code self}
     */
    static boolean containsAllImpl(Collection<?> self, Collection<?> c)
    {
        for (Object o : c)
        {
            if (!self.contains(o))
            {
                return false;
            }
        }
        return true;
    }

    /**
     * An implementation of {@link Collection#toString()}.
     */
    static String toStringImpl(final Collection<?> collection)
    {
        StringBuilder sb = newStringBuilderForCollection(collection.size()).append('[');
        boolean first = true;
        for (Object o : collection)
        {
            if (!first)
            {
                sb.append(", ");
            }
            first = false;
            if (o == collection)
            {
                sb.append("(this Collection)");
            }
            else
            {
                sb.append(o);
            }
        }
        return sb.append(']').toString();
    }

    /**
     * Returns best-effort-sized StringBuilder based on the given collection size.
     */
    static StringBuilder newStringBuilderForCollection(int size)
    {
        checkNonnegative(size, "size");
        return new StringBuilder((int) Math.min(size * 8L, Ints.MAX_POWER_OF_TWO));
    }

    /**
     * Returns a {@link Collection} of all the permutations of the specified {@link Iterable}.
     *
     * <p><i>Notes:</i> This is an implementation of the algorithm for Lexicographical Permutations
     * Generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7,
     * Section 7.2.1.2. The iteration order follows the lexicographical order. This means that the
     * first permutation will be in ascending order, and the last will be in descending order.
     *
     * <p>Duplicate elements are considered equal. For example, the list [1, 1] will have only one
     * permutation, instead of two. This is why the elements have to implement {@link Comparable}.
     *
     * <p>An empty iterable has only one permutation, which is an empty list.
     *
     * <p>This method is equivalent to {@code Collections2.orderedPermutations(list,
     * Ordering.natural())}.
     *
     * @param elements the original iterable whose elements have to be permuted.
     * @return an immutable {@link Collection} containing all the different permutations of the
     * original iterable.
     * @throws NullPointerException if the specified iterable is null or has any null elements.
     * @since 12.0
     */
    @Beta
    public static <E extends Comparable<? super E>> Collection<List<E>> orderedPermutations(
            Iterable<E> elements)
    {
        return orderedPermutations(elements, Ordering.natural());
    }

    /**
     * Returns a {@link Collection} of all the permutations of the specified {@link Iterable} using
     * the specified {@link Comparator} for establishing the lexicographical ordering.
     *
     * <p>Examples:
     *
     * <pre>{@code
     * for (List<String> perm : orderedPermutations(asList("b", "c", "a"))) {
     *   println(perm);
     * }
     * // -> ["a", "b", "c"]
     * // -> ["a", "c", "b"]
     * // -> ["b", "a", "c"]
     * // -> ["b", "c", "a"]
     * // -> ["c", "a", "b"]
     * // -> ["c", "b", "a"]
     *
     * for (List<Integer> perm : orderedPermutations(asList(1, 2, 2, 1))) {
     *   println(perm);
     * }
     * // -> [1, 1, 2, 2]
     * // -> [1, 2, 1, 2]
     * // -> [1, 2, 2, 1]
     * // -> [2, 1, 1, 2]
     * // -> [2, 1, 2, 1]
     * // -> [2, 2, 1, 1]
     * }</pre>
     *
     * <p><i>Notes:</i> This is an implementation of the algorithm for Lexicographical Permutations
     * Generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7,
     * Section 7.2.1.2. The iteration order follows the lexicographical order. This means that the
     * first permutation will be in ascending order, and the last will be in descending order.
     *
     * <p>Elements that compare equal are considered equal and no new permutations are created by
     * swapping them.
     *
     * <p>An empty iterable has only one permutation, which is an empty list.
     *
     * @param elements   the original iterable whose elements have to be permuted.
     * @param comparator a comparator for the iterable's elements.
     * @return an immutable {@link Collection} containing all the different permutations of the
     * original iterable.
     * @throws NullPointerException If the specified iterable is null, has any null elements, or if
     *                              the specified comparator is null.
     * @since 12.0
     */
    @Beta
    public static <E> Collection<List<E>> orderedPermutations(
            Iterable<E> elements, Comparator<? super E> comparator)
    {
        return new OrderedPermutationCollection<E>(elements, comparator);
    }

    private static final class OrderedPermutationCollection<E> extends AbstractCollection<List<E>>
    {
        final ImmutableList<E> inputList;
        final Comparator<? super E> comparator;
        final int size;

        OrderedPermutationCollection(Iterable<E> input, Comparator<? super E> comparator)
        {
            this.inputList = ImmutableList.sortedCopyOf(comparator, input);
            this.comparator = comparator;
            this.size = calculateSize(inputList, comparator);
        }

        /**
         * The number of permutations with repeated elements is calculated as follows:
         *
         * <ul>
         *   <li>For an empty list, it is 1 (base case).
         *   <li>When r numbers are added to a list of n-r elements, the number of permutations is
         *       increased by a factor of (n choose r).
         * </ul>
         */
        private static <E> int calculateSize(
                List<E> sortedInputList, Comparator<? super E> comparator)
        {
            int permutations = 1;
            int n = 1;
            int r = 1;
            while (n < sortedInputList.size())
            {
                int comparison = comparator.compare(sortedInputList.get(n - 1), sortedInputList.get(n));
                if (comparison < 0)
                {
                    // We move to the next non-repeated element.
                    permutations = IntMath.saturatedMultiply(permutations, IntMath.binomial(n, r));
                    r = 0;
                    if (permutations == Integer.MAX_VALUE)
                    {
                        return Integer.MAX_VALUE;
                    }
                }
                n++;
                r++;
            }
            return IntMath.saturatedMultiply(permutations, IntMath.binomial(n, r));
        }

        @Override
        public int size()
        {
            return size;
        }

        @Override
        public boolean isEmpty()
        {
            return false;
        }

        @Override
        public Iterator<List<E>> iterator()
        {
            return new OrderedPermutationIterator<E>(inputList, comparator);
        }

        @Override
        public boolean contains(@CheckForNull Object obj)
        {
            if (obj instanceof List)
            {
                List<?> list = (List<?>) obj;
                return isPermutation(inputList, list);
            }
            return false;
        }

        @Override
        public String toString()
        {
            return "orderedPermutationCollection(" + inputList + ")";
        }
    }

    private static final class OrderedPermutationIterator<E> extends AbstractIterator<List<E>>
    {
        @CheckForNull
        List<E> nextPermutation;
        final Comparator<? super E> comparator;

        OrderedPermutationIterator(List<E> list, Comparator<? super E> comparator)
        {
            this.nextPermutation = Lists.newArrayList(list);
            this.comparator = comparator;
        }

        @Override
        @CheckForNull
        protected List<E> computeNext()
        {
            if (nextPermutation == null)
            {
                return endOfData();
            }
            ImmutableList<E> next = ImmutableList.copyOf(nextPermutation);
            calculateNextPermutation();
            return next;
        }

        void calculateNextPermutation()
        {
            int j = findNextJ();
            if (j == -1)
            {
                nextPermutation = null;
                return;
            }
            /*
             * requireNonNull is safe because we don't clear nextPermutation until we're done calling this
             * method.
             */
            requireNonNull(nextPermutation);

            int l = findNextL(j);
            Collections.swap(nextPermutation, j, l);
            int n = nextPermutation.size();
            Collections.reverse(nextPermutation.subList(j + 1, n));
        }

        int findNextJ()
        {
            /*
             * requireNonNull is safe because we don't clear nextPermutation until we're done calling this
             * method.
             */
            requireNonNull(nextPermutation);
            for (int k = nextPermutation.size() - 2; k >= 0; k--)
            {
                if (comparator.compare(nextPermutation.get(k), nextPermutation.get(k + 1)) < 0)
                {
                    return k;
                }
            }
            return -1;
        }

        int findNextL(int j)
        {
            /*
             * requireNonNull is safe because we don't clear nextPermutation until we're done calling this
             * method.
             */
            requireNonNull(nextPermutation);
            E ak = nextPermutation.get(j);
            for (int l = nextPermutation.size() - 1; l > j; l--)
            {
                if (comparator.compare(ak, nextPermutation.get(l)) < 0)
                {
                    return l;
                }
            }
            throw new AssertionError("this statement should be unreachable");
        }
    }

    /**
     * Returns a {@link Collection} of all the permutations of the specified {@link Collection}.
     *
     * <p><i>Notes:</i> This is an implementation of the Plain Changes algorithm for permutations
     * generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7,
     * Section 7.2.1.2.
     *
     * <p>If the input list contains equal elements, some of the generated permutations will be equal.
     *
     * <p>An empty collection has only one permutation, which is an empty list.
     *
     * @param elements the original collection whose elements have to be permuted.
     * @return an immutable {@link Collection} containing all the different permutations of the
     * original collection.
     * @throws NullPointerException if the specified collection is null or has any null elements.
     * @since 12.0
     */
    @Beta
    public static <E> Collection<List<E>> permutations(Collection<E> elements)
    {
        return new PermutationCollection<E>(ImmutableList.copyOf(elements));
    }

    private static final class PermutationCollection<E> extends AbstractCollection<List<E>>
    {
        final ImmutableList<E> inputList;

        PermutationCollection(ImmutableList<E> input)
        {
            this.inputList = input;
        }

        @Override
        public int size()
        {
            return IntMath.factorial(inputList.size());
        }

        @Override
        public boolean isEmpty()
        {
            return false;
        }

        @Override
        public Iterator<List<E>> iterator()
        {
            return new PermutationIterator<E>(inputList);
        }

        @Override
        public boolean contains(@CheckForNull Object obj)
        {
            if (obj instanceof List)
            {
                List<?> list = (List<?>) obj;
                return isPermutation(inputList, list);
            }
            return false;
        }

        @Override
        public String toString()
        {
            return "permutations(" + inputList + ")";
        }
    }

    private static class PermutationIterator<E> extends AbstractIterator<List<E>>
    {
        final List<E> list;
        final int[] c;
        final int[] o;
        int j;

        PermutationIterator(List<E> list)
        {
            this.list = new ArrayList<E>(list);
            int n = list.size();
            c = new int[n];
            o = new int[n];
            Arrays.fill(c, 0);
            Arrays.fill(o, 1);
            j = Integer.MAX_VALUE;
        }

        @Override
        @CheckForNull
        protected List<E> computeNext()
        {
            if (j <= 0)
            {
                return endOfData();
            }
            ImmutableList<E> next = ImmutableList.copyOf(list);
            calculateNextPermutation();
            return next;
        }

        void calculateNextPermutation()
        {
            j = list.size() - 1;
            int s = 0;

            // Handle the special case of an empty list. Skip the calculation of the
            // next permutation.
            if (j == -1)
            {
                return;
            }

            while (true)
            {
                int q = c[j] + o[j];
                if (q < 0)
                {
                    switchDirection();
                    continue;
                }
                if (q == j + 1)
                {
                    if (j == 0)
                    {
                        break;
                    }
                    s++;
                    switchDirection();
                    continue;
                }

                Collections.swap(list, j - c[j] + s, j - q + s);
                c[j] = q;
                break;
            }
        }

        void switchDirection()
        {
            o[j] = -o[j];
            j--;
        }
    }

    /**
     * Returns {@code true} if the second list is a permutation of the first.
     */
    private static boolean isPermutation(List<?> first, List<?> second)
    {
        if (first.size() != second.size())
        {
            return false;
        }
        ObjectCountHashMap<?> firstCounts = counts(first);
        ObjectCountHashMap<?> secondCounts = counts(second);
        if (first.size() != second.size())
        {
            return false;
        }
        for (int i = 0; i < first.size(); i++)
        {
            if (firstCounts.getValue(i) != secondCounts.get(firstCounts.getKey(i)))
            {
                return false;
            }
        }
        return true;
    }

    private static <E> ObjectCountHashMap<E> counts(Collection<E> collection)
    {
        ObjectCountHashMap<E> map = new ObjectCountHashMap<>();
        for (E e : collection)
        {
            map.put(e, map.get(e) + 1);
        }
        return map;
    }
}
